The present invention is directed to variable capacitors and, more particularly, to double-bellows vacuum variable capacitors.
A known water-cooled vacuum variable capacitor 10 previously marketed by Jennings Technology, the owner of this patent, having a double-bellows configuration is shown partially in section in FIG. 1. The capacitor 10 generally included a variable end assembly 12 and a fixed end assembly 14 connected together by a body assembly 16. The end assemblies 12, 14 were typically fabricated from steel and, in some instances, were partially silver plated. The body assembly 16 was an insulator such as, for example, ceramic that mechanically coupled the end assemblies 12, 14 while keeping the end assemblies 12, 14 electrically insulated from one another.
Inside the capacitor 10 was a fixed can structure 20 that formed the first half of the capacitor 10. The second half of the capacitor 10 was formed by a variable can structure 22, which was mounted to a variable can plate 24. To change the capacitance of the capacitor 10, the variable can structure 22 and the can plate 24 were moved with respect to the fixed can structure 20 through the use of an adjustment mechanism 30.
A vacuum bellows 36 was used to seal the adjustment mechanism 30 from the rest of the capacitor 10. The vacuum bellows 36 was sealed to both the variable end assembly 12 and the variable can plate 24 so that any volume outside the vacuum bellows 36, shown generally as reference numeral 38 in FIG. 1, could be evacuated by attaching a vacuum source to one or both cap seals 40, 42.
To facilitate cooling, the capacitor 10 included a water jacket bellows 44. The water jacket bellows 44 was disposed between the vacuum bellows 36 and the adjustment mechanism 30 and was sealed between the variable can plate 24 and the variable end assembly 12. To cool the capacitor 10, water was circulated through the volume between the vacuum and water jacket bellows 36, 44 (shown generally as reference numeral 46), via inlet/outlet ports 50, 52.
Typically, the vacuum and water jacket bellows 36, 44 were fabricated from C510 phosphor bronze and had no perforations or holes therein because holes or perforations would either make it impossible to establish the vacuum or would allow water to escape from between the bellows 36, 44. As shown in FIG. 1, the bellows 36, 44 were convoluted, or corrugated, to allow the bellows 36, 44 to flex as the variable can plate 24 was moved.
The force required to move the can plate 24 was proportional to the product of the cross sectional area of vacuum bellows 36 and the pressure differential across the vacuum bellows 36. Additionally, the current carrying capacity of the capacitor 10 was directly proportional to the diameter of the vacuum bellows 36, because the vacuum bellows 36 carried the current in the capacitor 10. Accordingly, the more current that the capacitor 10 needed to carry, the more force it took to move the can plate 24 of the capacitor 10.
During operation, the variable end and fixed end assemblies 12, 14 were connected into a circuit requiring capacitance. Current would flow between the variable end assembly 12 and the fixed end assembly 14 through the bellows 36, 44, which connected the variable end assembly 12 to the variable can plate 24. The variable can plate 24 was, in turn, capacitively coupled to the fixed end assembly 14, via the fixed and variable can structures 20, 22. As the capacitor 10 was operated, water was circulated through the volume 46 between the bellows 36, 44, via the inlet/outlet ports 50, 52. Additionally, a motor was usually coupled to the adjustment mechanism 30 to tune the capacitor 10 by moving the variable can plate 24.